Improved catalytic hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks, such as petroleum refinery streams rich in lower olefins, for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst research, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing refinery streams that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. This process may supplant conventional alkylation units. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over H--ZSM--5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ aliphatic and aromatic hydrocarbons. Olefinic gasoline is produced in good yield by the MOGD process and may be recovered as a product or recycled to the reactor system for further conversion to distillate-range products.
As a consequence of the relatively low reactivity of ethene (ethylene) with known zeolite oligomerization catalysts prior distillate-mode reactor systems designed to completely convert a large ethylenic component of feedstock would require much larger size than comparable reactor systems for converting other lower olefins. However, under severe conditions of temperature and pressure, 75% or more of ethene can be converted in a single pass. Recycle of a major amount of ethene gas from the reactor effluent can result in significant increases in equipment size especially recycle compressors.
Olefinic feedstocks may be obtained from various sources, including fossil fuel processing streams, such as gas separation units, cracking of C.sub.2.sup.+ hydrocarbons, coal byproducts, alcohol or ether conversion, and various synthetic fuel processing streams. Cracking of ethane and conversion of effluent is disclosed in U.S. Pat. No. 4,100,218 and conversion of ethane to aromatics over Ga--ZSM--5 is disclosed in U.S. Pat. No. 4,350,835. Olefinic effluent from fluidized catalytic cracking of gas oil or the like is a valuable source of olefins suitable for exothermic conversion according to the present MOGD process. It has been found that an olefin-oligomerization process utilizing C.sub.2.sup.+ olefinic feedstock can be operated to fractionate the effluent for ethene recovery. Accordingly, it is an object of the present invention to provide a unique effluent fractionation system for recovery of unreacted ethylene or the like for operation of an integrated MOGD type reactor system.